The invention relates to a microfiltration device for the filtration of coagula and microaggregates from blood.
When blood is stored, microcoagula of an order of magnitude extending from less than 10 .mu.m to over 300 .mu.m form depending on such parameters as blood quality, blood stabilizer solution and method of storage.
The coagula consist mainly of thrombocytes, fibrinogen and other cellular and plasmatic components of the blood.
In the transfusion of preserved blood and blood components there is the danger that these coagula will be carried into the circulatory system, especially in the case of massive transfusions, and lodge primarily in the lung as the "physiological" filter. These coagula can then contribute to the effect of the so-called "wet lung syndrome" or to "transfusion lungs", i.e., to a blockage of the capillary vessels of the lungs. It is for this reason that so-called "transfusion filters" have been included in transfusion kits. These filters are simply screen or mesh filters with average pore sizes of 170 to 310 .mu.m. Thus, DIN 58,360 describes such a transfusion kit and at the same time calls for a surface area of 10 cm.sup.2 and a mesh opening of at least 310 .mu.m.
In recent years, special filters have been developed, so-called microaggregate or microcoagulum filters, which are capable of filtering coagula from the blood down to the 10 micrometer range. In these microcoagulum filters a distinction is made between sieve filters and depth filters. In depth filters, the filtration effect is based on a non-selective absorption of coagula by a layer of more or less densely packed fibers or by porous foam. Sieve filtration devices, on the other hand, operate with filters of a particular mesh size. There are also combinations of these filtration principles.
A number of requirements are generally established by the clinician for microcoagulum filters. These include a high filtration effectiveness of the unit, a high filtering capacity, a high flow rate, small priming volume, and easy handling of the unit.
Furthermore, such a filter must not produce any undesirable influence on the blood parameters, such as hemolysis etc., even at pressures as high as 400 mm of mercury, or in the administration of several units of preserved blood. Also, it must withstand manipulation, i.e., pressure transfusion, and plugging into other blood packages, without damage to filter and casing. On the other hand, it must be possible to produce it economically as a single-use unit.
In German Gebrauchsmuster No. 7,605,700, a microcoagulum filter is described in which the unit consists of a casing into which up to 5 filters, preferably at least 4 filters, in the form of sieves, are placed. These are in a cascade arrangement, i.e., the blood flows through the sieves one after the other, the next-following sieve having a higher filtering performance than the one before. In this manner a good filtration effect can be accomlished by using a relatively small filter surface area and by loading the filter surfaces selectively.
The device described in the above Gebrauchsmuster also has a drip chamber which is hermetically joined directly to the filtration chamber. The overall device is a sterilizable single-use unit. The sieves have a diminishing filter surface area combined with diminishing pore size, they are preferably constructed in truncoconical form, and they consist preferably of plain sieve filters whose smallest mesh size amount to 10 micrometers. The capacity of the unit amounts to at least 150 ml. Such a device is capable, for example, of filtering approximately five to ten ACD whole blood units stored for about two weeks, within 30 minutes under gravity conditions.
In spite of these relatively desirable properties, this device has often proven to be too large, both with regard to its manipulation and with regard to its capacity. To avoid unnecessary blood loss, the capacity of such a device should be as small as possible. Consequently, smaller devices are have been created. In German Gebrauchsmuster No. 7,923,865 there are described several modifications of the abovementioned device, in which only 2 to 3 filter elements of different characteristics are arranged in series, and which has a capacity of less than 150 ml. The performance of this device, however, has proven to be substantially poorer and its use is limited to the transfusion of single blood packages and erythrocyte concentrates without buffy coat (b.c.).
It is the object of the invention to provide a device which meets microcoagulum filter requirements, i.e., with which, for example, massive transfusions can be performed or preserved whole blood or b.c. erythrocyte concentrates can be filtered, but which has a small capacity combined with sufficient effectiveness and filtering ability.